Microelectronic semiconductor devices are often used in current control applications, such as power switching. For the most part, semiconductor power devices have replaced mechanical relays in power switching applications. Semiconductor devices which are suitable for power switching applications include bipolar junction transistors, junction field effect transistors, insulated gate field effect transistors and, in particular, semiconductor controlled rectifiers sometimes referred to as "thyristors".
The suitability of a semiconductor power device for a particular application depends primarily on the electrical characteristics of the device and on the demands of the circuit. It may be necessary for the device to provide a specified minimum switching speed or minimum blocking voltage in order to properly function in the circuit. One important consideration in some applications is whether the device can be easily integrated with other elements of an integrated circuit. An integrated circuit or "IC" is an assembly of interconnected passive and active circuit elements formed in a crystalline "chip" of semiconductor material. Batch processing allows a large number of identical chips to be fabricated at low cost using a sequence of doping, masking and etching techniques. Both the design of a device and its fabrication sequence are important in determining its suitability for integration with other circuit elements.
Power switching in an automotive context generally involves low voltages, usually 10 to 100 volts dc, at currents of about 10 to 100 amperes. For automotive applications, it is desirable to provide a switch that can be controlled by low positive voltages and low current and which has a grounded cathode for compatibility with typical automotive circuitry. It is also desirable to provide a power switch having a low "on" resistance and high current density to reduce cost. Thyristors possess many of these attributes and thus are of interest in automotive circuit design.
Generally, thyristors have designated regions which are doped to provide a multi-layer pnpn structure. In the on-state, thyristors are characterized by regenerative transistor action. In operation, the pnpn structure of a thyristor is electrically analogous to a combination of a pnp transistor and an npn transistor. When the emitter-base junction of the pnp structure is forward biased by an appropriate voltage, the pnp transistor is turned on. The transistors are arranged such that the pnp transistor action provides holes for injection into the base of the npn transistor across the latter's emitter-base junction which is also forward biased. The hole current created by the pnp transistor action supplies base drive to the npn transistor so that the npn transistor is turned on. When the sum of the current gain alpha of the two transistors exceeds unity, regenerative switching occurs and the thyristor is "latched" on. In a gate controlled thyristor, turn-on is initiated by the action of one or more insulated gate structures.
In my prior U.S. Pat. applications, "Integrated Field Controlled Thyristor Structure with Grounded Cathode", Ser. No. 617,106, filed June 4, 1984, now U.S. Pat. No. 4,611,235; "Insulated Gate Controlled Thyristor", Ser. No. 667,845, filed Nov. 2, 1984, now U.S. Pat. No. 4,630,092; and "Insulated Gate Controlled Thyristor Having Shorted Anode", Ser. No. 667,827, filed Nov. 2, 1984, now U.S. Pat. No. 4,636,830; I disclose novel vertical thyristors having multiple insulated gate electrodes which turn the thyristors on and off. Lateral examples of such thyristors are only incidentally disclosed. These dual gate thyristors are especially suitable for electrical switching in automotive applications. Moreover, turn-off in my thyristors is achieved more readily by virtue of a junction field effect "pinch resistance" which restricts current flow to accelerate device turn-off. However, in some applications, a vertical thyristor structure may be less desirable than a lateral structure for integration with other circuit elements. Generally, the integration of control and logic devices with a vertical thyristor structure requires that these circuit elements be formed in portions of the thyristor body. However, in operation the thyristor body always has some electrical potential. That is, in both the forward-blocking state and the forward-conducting state, portions of the thyristor body are electrically charged. Therefore, when other circuit elements such as logic and control structures are formed in the thyristor body, these existing charges must be taken into consideration. Consequently, it would be desirable to provide a dual gate thyristor which is electrically isolated in a semiconductor layer such that it can be integrated with logic and control structures. I have discovered such a device.